1. Field of the Invention
The present invention relates to a stand-alone transmission controller, and particularly to a stand-alone transmission controller applicable to a stand-alone data transmission system in which transmission control stations are linked to form a loop and the data transmission control functions are distributed among the stations.
2. Description of the Prior Art
Recent data communication systems do not entirely rely on general-purpose means such as the telephone exchange network, but they now avail themselves of data exchange networks more sophisticated in all aspects of function, performance and economy. Connecting systems for making linkage between communication lines which constitute the above-mentioned data exchange networks and exchange units (exchange stations), concentrator-allotters, terminal units and the like located at the end of the lines include the exchange connection system, branch connection system, concentrator-allotter connection system, and loop connection system.
Among the various connecting systems, the loop connection system in which a high-speed transmission line connects terminal units and computers in the form of a loop is applicable to the connection between a central computer and terminal units and also to the mutual connection among arbitrary terminal units.
A conventional data transmission controller using the loop connection system is disclosed in Japanese Patent Publication No. 46-35931 (filed on Oct. 21, 1971) entitled "Method and apparatus for information transmission in a data transmission system". The system configuration by this prior art method includes, as shown in FIGS. 1 and 2, a main pulse transmitter-receiver 1 provided in the central computer, a modulator-demodulator (will be termed simply "modem" hereinafter) 2 provided in the main pulse transmitter-receiver 1 and made up of a modulator 2a and a demodulator 2b, a plurality of terminal units 4, 6, 8 and 10 located at the end of the transmission line and each made up of a switching unit 3 and a pulse transmitter-receiver 5 (7, 9, 11) connected to the switching unit 3, and terminal modems 13, 14, 15 and 16 connected in series to a transmission line 12 and each provided for a group of serially connected terminal units 4 (6, 8, 10). The terminal modem 13 (14, 15, 16) consists of a modulator 13a (14a, 15a, 16a) and a demodulator 13b (14b, 15b, 16b).
As shown in more detail in FIG. 2, the switching unit 3 consists of a pulse register 17, a decoder 18, a first terminal 19 for shunting the pulse register 17, a second terminal 20 for input connected to the pulse register 17, a third terminal 21 connected to the pulse transmitter-receiver 5 (7, 9, 11), and a switching means 22 which selects one of the first, second and third terminals 19, 20 and 21.
Next, the operation of the foregoing conventional transmission controller will be described. The closedloop data transmission system shown in FIG. 1 has three operating modes. These are the first mode in which all switching units 3 are shunted to make the terminal lines in a blank state, the second mode in which information is transferred from the pulse transmitter-receivers 5, 7, 9 and 11 to the main pulse transmitter-receiver 1, and the third mode in which information is transferred from the main pulse transmitter-receiver 1 to the terminal pulse transmitter-receivers 5, 7, 9 and 11.
In the first mode, the switching means 22 selects the first terminal 19 for shunting in all switching units 3 in FIG. 2, and the main pulse transmitter-receiver 1 transmits a series of characters each made up of a pulse combination A. Since all switching units 3 are shorted at their input and output terminals, the output pulses from all demodulators 13b, 14b, 15b and 16b are conducted directly to the respective modulators 13a, 14a, 15a and 16a, and the flow of characters transmitted by the main pulse transmitter-receiver 1 is reproduced in each modem 13, 14, 15 and 16, and eventually it returns via the demodulator 2b to the main pulse transmitter-receiver 1. Making the character flow has two purposes. The first aim is to establish the synchronism between the oscillator in a demodulator, e.g., 14b, and the oscillator in a preceding modulator, e.g., 13a, and the second aim is to establish the synchronism between the pulse train transmitted as characters by the main pulse transmitter-receiver 1 and the pulse train detected as characters by the pulse register 17. When a character received by the pulse transmitter-receiver 1 via the demodulator 2b has the same pulse combination as a character transmitted by the pulse transmitter-receiver 1, all units in the data transmission system are proved to be synchronous.
Next, the second mode in which the main pulse transmitter-receiver 1 is ready to receive information from pulse transmitter-receivers 5, 7, 9 and 11 in the terminal units 4, 6, 8 and 10 will be described. The main pulse transmitter-receiver 1 suspends the transmission of character A and transmits two characters B and C, and thereafter resumes the transmission for character A. The purpose of transmitting the characters B and C is as follows. Once the decoder 18 has recognized character B in the pulse register 17, a pertinent pulse transmitter-receiver 5, 7, 9 or 11 having information to be sent commences the following process. 1 The switching means 22 is set to select the second terminal 20; 2 The pulse register contents are checked to test if the following character is C; 3 If the character is found to be C the switching means 22 is set to the third terminal 21 so that only characters transmitted from a pertinent pulse transmitter-receiver pass. Therefore, the character C is prevented from advancing to the next terminal unit 4, 6, 8 or 10 in the serial linkage, and consequently the next pulse register 17 does not receive character C immediately following the reception of character B. If a terminal unit 4, 6, 8 or 10 has no information to be sent when character B is detected by the pulse register 17, the switching means 22 stays at the vacant position. Information transmitted from any of the pulse transmitter-receivers 5, 7, 9 and 11 goes through the switching means 22 in the next unit in the serial linkage and arrives at the entry of the main pulse transmitter-receiver 1. Each of the pulse transmitter-receivers 5, 7, 9 and 11 terminates the transmission with characters B and C and resets the switching means 22 to the first terminal 19, i.e., the vacant position. Then, the above process is repeated by the next terminal unit.
Reception of characters B and C by the main pulse transmitter-receiver 1 provides the opportunity of information transmission for all pulse transmitter-receivers 5, 7, 9 and 11 in the closed loop, and this is indicated by the main pulse transmitter-receiver 1 through the transmission of character D which signifies that information sent from a low-ranking terminal unit has been received normally. If, on the other hand, information received by the main pulse transmitter-receiver 1 includes error, the above process will be retried by transmitting characters B and C again. Any pulse transmitter-receiver 5, 7, 9 or 11 having no reception of character D despite the transmission of information will send the information again.
Finally, the third mode in which the main pulse transmitter-receiver 1 is ready to send information to any of the pulse transmitter-receivers 5, 7, 9 and 11 will be described. Information transmission in this mode is implemented by transmitting two characters D and X in advance of each message. The character D signifies that the subsequent character X addresses a specific pulse transmitter-receiver intended for the message. The message goes through all switching units 3, but only a pulse transmitter-receiver with assignment of character X among pulse transmitter-receivers 5, 7, 9 and 11 will respond to the information.
However, the foregoing conventional transmission controller, in which information transmission and reception take place always via the main pulse transmitter-receiver, has a major problem that if the main pulse transmitter-receiver fails, communication among all terminal units is shut down.
Another problem is that the transmission controller implementing transmission entirely via the main pulse transmitter-receiver allows only one-to-N stations communication, and achieves a low efficiency when intended to have N-to-N stations communication.